1. Field of the Invention
The invention relates to a high pressure mercury lamp. The invention relates especially to a super-high pressure mercury lamp in which a discharge vessel is filled with mercury in an amount greater than or equal to 0.16 mg/mm3, in which furthermore the mercury vapor pressure during operation is greater than or equal to a hundred and some dozen atm, and which is used as a backlight of a liquid crystal display device or the like.
2. Description of Related Art
In a liquid crystal display device of the projection type there is a need for uniform illumination of images onto a rectangular screen with adequate color reproduction.
Therefore, a metal halide lamp which is filled with mercury and a metal halide is used as the light source. Furthermore, smaller and smaller metal halide lamps, and more and more often point light sources have recently been produced, and lamps with extremely small distances between the electrodes have been used in practice.
Against this background, instead of metal halide lamps, lamps with an extremely high mercury vapor pressure, for example, with a vapor pressure greater than or equal to 200 bar (roughly 197 atm), have recently been proposed. Here, the increased mercury vapor pressure suppresses broadening of the arc (the arc is contracted) and a considerable increase of the light intensity is desired; this is disclosed, for example, in Japanese patent disclosure document HEI 2-148561 (U.S. Pat. No. 5,109,181) and in Japanese patent disclosure document HEI 6-52830 (U.S. Pat. No. 5,497,049).
Japanese patent disclosure document HEI2-148561 (U.S. Pat. No. 5,109,181) discloses a high pressure mercury lamp in which a discharge vessel which has a pair of tungsten electrodes is filled with a rare gas, greater than or equal to 0.2 mg/mm3 mercury and halogen in the range from 1xc3x9710xe2x88x926 to 1xc3x9710xe2x88x924 xcexcmol/mm3, and which is operated with a wall load of greater than or equal to 1 W/mm2.
The reason for the amount of mercury added being greater than or equal to 0.2 mg/mm3 is to raise the mercury pressure, to multiply the continuous spectrum in the visible radiation range, especially in the red range, and to improve the color reproduction. The reason for the wall load of greater than or equal to 1 W/mm2 is to increase the temperature in the coolest portion in order to increase the mercury pressure.
On the other hand, Japanese patent disclosure document HEI 6-52830 (U.S. Pat. No. 5,497,049) discloses that, in addition to the above described amount of mercury, the value of the wall load, the amount of halogen, the shape of the discharge vessel, and the distance between the electrodes are fixed.
It is stated that this lamp is suited for a light source for a projector. Furthermore, it is described herein that the degree of maintenance of the screen illuminance is greater than when using a conventional lamp.
The lamps disclosed in the above described publications of the prior art each have a foil seal arrangement which is shown in FIG. 6. In the figure, a discharge vessel 61 has a hermetically sealed portion 62 formed on each of its two ends and in which a metal foil 63 made of molybdenum or the like is inserted. An electrode 64 is connected to the side of the respective metal foil 63 which points into the discharge vessel 61, while an outer lead 65 is connected to its side which points toward the exterior. FIG. 7 is a cross section taken along line 7xe2x80x947 in FIG. 6. Between the metal foil 63 and the electrode 64, a gap S inevitably forms.
Since this mercury lamp has a very high internal pressure of one hundred and some dozen atmospheres during operation, as was described above, cracks often form leading from the gap S; this also leads to damaging of the lamp and shortening of the operating service life.
Therefore, a primary object of the present invention is to devise a super-high pressure mercury lamp with an internal pressure during operation of one hundred and some dozen atm, which has a new arrangement in which in the hermetically sealed portions neither damage the lamp nor shorten its operating service life.
In a high pressure mercury lamp in which a silica glass discharge vessel contains a pair of electrodes opposite one another, which is filled with mercury in an amount of greater than or equal to 0.16 mg/mm3 and rare gas, and in which the wall load is greater than or equal to 0.8 W/mm2, the above object is achieved in accordance with the invention in that a side tube part is formed on each end of this discharge vessel which has a sealing body formed of a functional gradient material which is formed essentially of a dielectric material and an electrically conductive material with a ratio to one another which changes in the lengthwise direction of the sealing body, and which has a dielectric end area and an electrically conductive end area, and in that the sealing body in its dielectric end area is connected to the side tube and is connected to the electrode at its electrically conductive end area.
The object is furthermore advantageously achieved according to the invention when the high pressure mercury lamp has the above described arrangement and the condition a/b xe2x89xa60.25 is met, where a is the value of the outside diameter on the end face of the sealing body on the side of the discharge vessel and b is the value of the outside diameter of the side tube in this area.
In the following the invention is further described using several embodiments which are shown in the drawings.